Attempts at controlling dynamic, continuous processes, including polyolefin production processes have been a long standing goal of the process industry.
Gas phase processes for the homopolymerization and copolymerization of monomers, especially olefin monomers, are well known in the art. Such processes can be conducted, for example, by introducing the gaseous monomer or monomers into a stirred and/or fluidized bed of resin particles and catalyst.
In the fluidized-bed polymerization of olefins, the polymerization is conducted in a fluidized-bed reactor, wherein a bed of polymer particles is maintained in a fluidized state by means of an ascending gas stream including gaseous reaction monomer. The polymerization of olefins in a stirred-bed reactor differs from polymerization in a gas fluidized-bed reactor by the action of a mechanical stirrer within the reaction zone, which contributes to fluidization of the bed. As used herein, the term “fluidized-bed” also includes stirred-bed processes and reactors.
The start-up of a fluidized bed reactor generally uses a bed of pre-formed polymer particles. During the course of polymerization, fresh polymer is generated by the catalytic polymerization of the monomer, and polymer product is withdrawn to maintain the bed at constant volume. An industrially favored process employs a fluidization grid to distribute the fluidizing gas to the bed, and also to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via one or more discharge conduits disposed in the lower portion of the reactor, near the fluidization grid. The fluidized bed includes a bed of growing polymer particles, polymer product particles and catalyst particles. This reaction mixture is maintained in a fluidized condition by the continuous upward flow from the base of the reactor of a fluidizing gas which includes recycle gas drawn from the top of the reactor, together with added make-up monomer. The fluidizing gas enters the bottom of the reactor and is passed through a fluidization grid, upwardly through the fluidized bed.
A variety of gas phase polymerization processes are known. For example, the recycle stream can be cooled to a temperature below the dew point, resulting in condensing a portion of the recycle stream, as described in U.S. Pat. Nos. 4,543,399 and 4,588,790. This intentional introduction of a liquid into a recycle stream or reactor during the process is referred to generally as a “condensed mode” operation.
Further details of fluidized bed reactors and their operation are disclosed in, for example, U.S. Pat. Nos. 4,243,619, 4,543,399, 5,352,749, 5,436,304, 5,405,922, 5,462,999, and 6,218,484, the disclosures of which are incorporated herein by reference.
For example, U.S. Pat. No. 5,525,678 suggests a catalyst including a zirconium metallocene that produces a relatively low molecular weight, high comonomer-content polymer, and a titanium non-metallocene that produces a relatively high molecular weight, low comonomer-content polymer. Typically, ethylene is the primary monomer, and small amounts of hexene or other alpha-olefins are added to lower the density of the polyethylene. The zirconium catalyst incorporates most of the comonomer and hydrogen, so that, in a typical example, about 85% of the hexene and 92% of the hydrogen are in the low molecular weight polymer. Water is added to control the overall molecular weight by controlling the activity of the zirconium catalyst.
When polymerizing with two or more catalysts, it is desirable to monitor and control the relative contribution of each catalyst to the polymer product, so that the polymerization conditions can be adjusted to obtain the desired polymer properties. The properties of the polymer produced in the reactor are affected by a variety of operating parameters, such as reaction temperature, monomer feed rates, catalyst feed rates, co-catalyst feed rates, hydrogen gas concentration, or water feed rate. In order to produce polymer having a desired set of properties, polymer exiting the reactor is sampled and laboratory measurements carried out to characterize the polymer. If it is discovered that one or more polymer properties are outside a desired range, polymerization conditions can be adjusted, and the polymer resampled. This periodic sampling, testing and adjusting, however, is undesirably slow, since sampling and laboratory testing of polymer properties is time-consuming. As a result, conventional processes can produce large quantities of “off-specification” polymer before manual testing and reactor control can effectively adjust the polymerization conditions.
In WO 03/044061 a rolling average of a ratio of two gas phase component concentrations, each concentration in turn expressed as a component's gas phase mole fraction divided by its feed rate into the reactor, as seen in equation (7) on page 13 of that publication, is referred to as a LI. The LI gives an indication of the polymer properties being produced, without waiting for manual product analysis. This technique results in improved control compared to the prior art. However, using the technique of WO 03/044061 yields somewhat inaccurate control when feed ratios change and also the equation did not consider methods to choose the leading indicator target. Further, improvements made in industrial use applied a leading indicator based on a weighting factor in turn based on polymer residence time in the reactor. While using this latter method, the leading indicator based on polymer residence time, gave a better indication of reactor behavior than the rolling average of WO 03/044061, a quicker, more responsive leading indicator was sought. Such a more responsive leading indicator might allow improved, more timely control of polymerization processes, and in so doing, permit reduction of off-test or off-specification polymer in response to either unintended reactor variable perturbations, or minimizing such off-test or off-specification polymer in response to an intended change in reactor variables, such as when changing such variables to achieve a different class of material by effecting a change to polymer properties such as melt index, flow index, density, molecular weight, molecular weight distribution or combinations thereof by adjusting reactor variables.
Thus, it would be desirable to have faster methods and more accurate methods for monitoring and/or predicting changes in polymer properties, or changes in relative activities of catalysts, especially in multiple catalyst processes. In addition, it would be desirable to have methods to predict what reactor conditions would be required, based on simple reactor data and product properties, to produce a particular type or kind of polymer product. It is especially important to minimize the production of polymer product that does not meet desired specifications during times when the reactor process conditions are changing (either deliberately or through process parameter drift).